latex: improve checks when build jobs are skipped

[haraldschilly]

e.g. in run_sagetex, the this._last_sagetex_hash = hash is set in the finally block.

Therefore, if sagetex is interrupted, the hash is set anyways, and the next time around sagetex is not run again. … But I think it would be more natural, to attempt a failed sagetex run again the next time around. Right now, it is only attempted again if temp files are cleared, a force-build is launched, or well, the content to evaluate changes.

Probably the same needs to be sorted out for knitr and pythontex.